This is one of the common allegations that radical animal right groups often make against bullfighting and its promoters. The idea is of course to play into the common misconeption of the corrida being a fight between man and beast and an unfair one at that.
First, the corrida cannot be seen as a sport in modern anglo-saxon or northern european sense. Second, drugging the bull, for example putting vaseline in his eyes or feeding him sedatives would be directly counterproductive to the spectacle.
The Matador has to trust that the bull has an intact sense of vision in order to keep him engaged in the lure of the cape. If the animal‘s vision was blurry or otherwise affected, this would increase the risk for the Matador as it would make it harder to focus the bull on a single visual edge. Every bull has to be cleared by a vet before entering the ring, injured, weakened or bulls with any sort of defect will not pass this examination. The crowd expects a lively and active bull, if a bull is not charging and generally comes across as timid it is often returned to the pens by petition of the crowd and subsequently replaced by a new bull.
Bulls arriving in the holding pen of the Las Ventas bullring in Madrid